ConcurrentLinkedDeque

An unbounded concurrent deque based on linked nodes.
Concurrent insertion, removal, and access operations execute safely
across multiple threads.
A ConcurrentLinkedDeque is an appropriate choice when
many threads will share access to a common collection.
Like most other concurrent collection implementations, this class
does not permit the use of null elements.

Beware that, unlike in most collections, the size method
is NOT a constant-time operation. Because of the
asynchronous nature of these deques, determining the current number
of elements requires a traversal of the elements, and so may report
inaccurate results if this collection is modified during traversal.
Additionally, the bulk operations addAll,
removeAll, retainAll, containsAll,
equals, and toArray are not guaranteed
to be performed atomically. For example, an iterator operating
concurrently with an addAll operation might view only some
of the added elements.

This class and its iterator implement all of the optional
methods of the Deque and Iterator interfaces.

Memory consistency effects: As with other concurrent collections,
actions in a thread prior to placing an object into a
ConcurrentLinkedDequehappen-before
actions subsequent to the access or removal of that element from
the ConcurrentLinkedDeque in another thread.

Pushes an element onto the stack represented by this deque (in other
words, at the head of this deque) if it is possible to do so
immediately without violating capacity restrictions, throwing an
IllegalStateException if no space is currently available.

Causes the current thread to wait until another thread invokes the
notify() method or the
notifyAll() method for this object, or
some other thread interrupts the current thread, or a certain
amount of real time has elapsed.

Inserts the specified element into the queue represented by this deque
(in other words, at the tail of this deque) if it is possible to do so
immediately without violating capacity restrictions, returning
true upon success and throwing an
IllegalStateException if no space is currently available.

Inserts the specified element at the front of this deque if it is
possible to do so immediately without violating capacity restrictions,
throwing an IllegalStateException if no space is currently
available.

Inserts the specified element at the end of this deque if it is
possible to do so immediately without violating capacity restrictions,
throwing an IllegalStateException if no space is currently
available.

Inserts the specified element into the queue represented by this deque
(in other words, at the tail of this deque) if it is possible to do so
immediately without violating capacity restrictions, returning
true upon success and false if no space is currently
available.

Pushes an element onto the stack represented by this deque (in other
words, at the head of this deque) if it is possible to do so
immediately without violating capacity restrictions, throwing an
IllegalStateException if no space is currently available.

Inserts the specified element into this queue if it is possible to do so
immediately without violating capacity restrictions, returning
true upon success and throwing an IllegalStateException
if no space is currently available.

addAll

Appends all of the elements in the specified collection to the end of
this deque, in the order that they are returned by the specified
collection's iterator. Attempts to addAll of a deque to
itself result in IllegalArgumentException.

element

Retrieves, but does not remove, the head of the queue represented by
this deque (in other words, the first element of this deque).
This method differs from peek only in that it throws an
exception if this deque is empty.

push

Pushes an element onto the stack represented by this deque (in other
words, at the head of this deque) if it is possible to do so
immediately without violating capacity restrictions, throwing an
IllegalStateException if no space is currently available.

remove

Retrieves and removes the head of the queue represented by this deque
(in other words, the first element of this deque).
This method differs from poll only in that it throws an
exception if this deque is empty.

remove

Removes the first occurrence of the specified element from this deque.
If the deque does not contain the element, it is unchanged.
More formally, removes the first element e such that
o.equals(e) (if such an element exists).
Returns true if this deque contained the specified element
(or equivalently, if this deque changed as a result of the call).

removeFirstOccurrence

Removes the first occurrence of the specified element from this deque.
If the deque does not contain the element, it is unchanged.
More formally, removes the first element e such that
o.equals(e) (if such an element exists).
Returns true if this deque contained the specified element
(or equivalently, if this deque changed as a result of the call).

removeLastOccurrence

Removes the last occurrence of the specified element from this deque.
If the deque does not contain the element, it is unchanged.
More formally, removes the last element e such that
o.equals(e) (if such an element exists).
Returns true if this deque contained the specified element
(or equivalently, if this deque changed as a result of the call).

size

Returns the number of elements in this deque. If this deque
contains more than Integer.MAX_VALUE elements, it
returns Integer.MAX_VALUE.

Beware that, unlike in most collections, this method is
NOT a constant-time operation. Because of the
asynchronous nature of these deques, determining the current
number of elements requires traversing them all to count them.
Additionally, it is possible for the size to change during
execution of this method, in which case the returned result
will be inaccurate. Thus, this method is typically not very
useful in concurrent applications.

Implementation Note:

toArray

Returns an array containing all of the elements in this deque, in
proper sequence (from first to last element).

The returned array will be "safe" in that no references to it are
maintained by this deque. (In other words, this method must allocate
a new array). The caller is thus free to modify the returned array.

This method acts as bridge between array-based and collection-based
APIs.

toArray

Returns an array containing all of the elements in this deque,
in proper sequence (from first to last element); the runtime
type of the returned array is that of the specified array. If
the deque fits in the specified array, it is returned therein.
Otherwise, a new array is allocated with the runtime type of
the specified array and the size of this deque.

If this deque fits in the specified array with room to spare
(i.e., the array has more elements than this deque), the element in
the array immediately following the end of the deque is set to
null.

Like the toArray() method, this method acts as
bridge between array-based and collection-based APIs. Further,
this method allows precise control over the runtime type of the
output array, and may, under certain circumstances, be used to
save allocation costs.

Suppose x is a deque known to contain only strings.
The following code can be used to dump the deque into a newly
allocated array of String:

String[] y = x.toArray(new String[0]);

Note that toArray(new Object[0]) is identical in function to
toArray().

Parameters

a

T: the array into which the elements of the deque are to
be stored, if it is big enough; otherwise, a new array of the
same runtime type is allocated for this purpose

toString

Returns a string representation of this collection. The string
representation consists of a list of the collection's elements in the
order they are returned by its iterator, enclosed in square brackets
("[]"). Adjacent elements are separated by the characters
", " (comma and space). Elements are converted to strings as
by valueOf(Object).